Apparatus and Method for Angle Rotation,xe2x80x9d U.S. patent application Ser. No. 09/698,246, filed Oct. 30, 2000; and
Apparatus and Method for Rectangular-to-Polar Conversion,xe2x80x9d U.S. patent application Ser. No. 09/698,249, filed Oct. 30, 2000.
1. Field of the Invention
The present invention is related to digital signal processing and digital communications. More specifically the present invention is related to interpolation, angle rotation, rectangular-to-polar conversion, and carrier and symbol timing recovery for digital processing and digital communications applications.
2. Related Art
Advances in technology have enabled high-quality, low-cost communications with global coverage, and provide the possibility for fast Internet access and multimedia to be added to existing services. Exemplary emerging technologies include cellular mobile radio and digital video broadcasting, both of which are described briefly as follows.
In recent years, cellular mobile radio has experienced rapid growth due to the desire for mobility while enjoying the two-way voice services it provides. GSM, IS-136 and personal digital cellular (PDC) are among the most successful second-generation personal communications (PCS) technologies in the world today, and are responsible for providing cellular and PCS services globally. As the technology advances, customers will certainly demand more from their wireless services. For example, with the explosive growth of the world wide web over the wired networks, it is desirable to provide Internet services over mobile radio networks. One effort to specify the future global wireless access system is known as IMT-2000 (Buchanan, K., et al., IEEE Pers. Comm. 4:8-13 (1997)). The goal of IMT-2000 is to provide not only traditional mobile voice communications, but also a variety of voice and data services with a wide range of applications such as multimedia capabilities, Internet access, imaging and video conferencing. It is also an aim to unify many existing diverse systems (paging, cordless, cellular, mobile satellite, etc.) into a seamless radio structure offering a wide range of services. Another principle is to integrate mobile and fixed networks in order to provide fixed network services over the wireless infrastructure. Such systems might well utilize broadband transport technologies such as a synchronous transfer mode (ATM).
For the applications of IMT-2000, a high-bit-rate service is needed. Moreover, for multimedia applications, the system should provide a multitude of services each requiring 1) a different rate, and 2) a different quality-of-service parameter. Thus, a flexible, variable-rate access with data rates approaching 2 Mb/s is proposed for IMT-2000.
The advent of digital television systems has transformed the classical TV channel into a fast and reliable data transmission medium. According to the specifications of the DVB project (Reimers, U., IEEE Comm. Magazine 36:104-110 (1998)), digital TV is no longer restricted to transmitting sound and images but instead has become a data broadcasting mechanism which is fully transparent to all contents. Digital TV broadcasting by satellite, cable and terrestrial networks is currently under intensive development. A typical system looks like this: a DVB signal is received from a satellite dish, from cable, or from an antenna (terrestrial reception). A modem built into an integrated receiver/decoder (IRD) will demodulate and decode the signal. The information received will be displayed on a digital TV or a multimedia PC. In addition to being used as a digital TV, DVB can receive data streams from companies who wish to transmit large amounts of data to many reception sites. These organizations may be banks, chains of retail stores, or information brokers who wish to offer access to selected Internet sites at high data rates. One such system is MultiMedia Mobile (M3), which has a data rate of 16 Mb/s.
For proper operation, these third generation systems require proper synchronization between the transmitter and the receiver. More specifically, the frequency and phase of the receiver local oscillator should substantially match that of the transmitter local oscillator. When there is a mismatch, then an undesirable rotation of the symbol constellation will occur at the receiver, which will seriously degrade system performance. When the carrier frequency offset is much smaller than the symbol rate, the phase and frequency mismatches can be corrected at baseband by using a phase rotator. It is also necessary to synchronize the sampling clock such that it extracts symbols at the correct times. This can be achieved digitally by performing appropriate digital resamples resampling.
The digital resampler and the direct digital frequency synthesizer (DDS) used by the phase rotator are among the most complex components in a receiver (Cho, K., xe2x80x9cA frequency-agile single-chip QAM modulator with beamforming diversity,xe2x80x9d Ph.D. dissertation, University of California, Los Angeles (1999)). Their performance is significant in the overall design of a communications modem. For multimedia communications, the high-data-rate requirement would impose a demand for high computational power. However, for mobile personal communication systems, low cost, small size and long battery life are desirable. Therefore, it would be desirable to have an efficient implementation of the phase rotator, re-sampler, and DDS in order to perform fast signal processing that operates within the available resources. Furthermore, it would be desirable to have an efficient synchronization mechanism that uses a unified approach to timing and carrier phase corrections.
For Internet services it is important to provide instantaneous throughput intermittently. Packet data systems allow the multiplexing of a number of users on a single channel, providing access to users only when they need it. This way the service can be made more cost-effective. However, the user data content of such a transmission is usually very short. Therefore, it is essential to acquire the synchronization parameters rapidly from the observation of a short signal-segment.
For applications where low power and low complexity are the major requirements, such as in personal communications, it is desirable to sample the signal at the lowest possible rate, and to have a synchronizer that is as simple as possible. Therefore, it is also desirable to have an efficient synchronizer architecture that achieves these goals.
For applications utilizing Orthogonal Frequency Division Multiplexing (OFDM), sampling phase shift error produces a rotation of the Fast Fourier Transform (FFT) outputs (Pollet T., and Peters, M., IEEE Comm. Magazine 37:80-86 (1999)). A phase correction can be achieved at the receiver by rotating the FFT outputs. Therefore, it is also desirable to have an efficient implementation structure to perform rotations of complex numbers.
The present invention is directed at high-performance and low-cost methods and apparatus for performing an accurate interpolation operation in a digital device for generating an output signal. Applications include digital re-sampling to change the data sampling rate of a given signal, and interpolation to correct for symbol timing errors in a digital communications device, as well as other applications.
More specifically, the present invention includes a method and apparatus for trigonometric interpolation that interpolates between two data samples at an offset xcexc, where the two data samples are part of a set of N-data samples. The trigonometric interpolator fits a trigonometric polynomial to the N-data sample set, and then evaluates the trigonometric polynomial at the offset xcexc. The trigonometric interpolator can be implemented for any number of data samples N, where the accuracy of the interpolation (and the hardware complexity) increases with the number of data samples. Simulations have shown that the trigonometric interpolator achieves superior interpolation performance over other methods of interpolation. Moreover, the trigonometric interpolator generally requires less hardware than other methods of interpolation.
The trigonometric interpolator can be implemented using a delay module, an adder/subtractor module, and at least one angle rotator. The delay module and adder/subtractor module generate one or more complex trigonometric coefficients based on the input samples. Each angle rotator rotates a corresponding complex trigonometric coefficient in the complex plane according the offset xcexc. The angle rotator can be implemented using a lookup table (e.g. memory device) or by using an angle rotator processor. The table lookup approach is advantageous for low latency applications and can be efficiently shared with a phase rotator that performs carrier synchronization. The angle rotator processor is advantageous for applications that require low complexity, and or low power requirements.
In embodiments, the number of angle rotators that are required to implement the trigonometric interpolation is reduced for a given set of N-data samples. This is accomplished by modifying data samples so that the trigonometric coefficient cN/2 is zero, thereby eliminating the need for the corresponding angle rotator. The hardware xe2x80x9cpricexe2x80x9d for eliminating an angle rotator using this method is an additional multiplier.
In embodiments, the filter response of the interpolator can be modified to achieve an arbitrary frequency response in order to enhance the interpolator performance. More specifically, the frequency response of the interpolator can be shaped to effectively correspond with the frequency response of the input data samples and the offset xcexc. In other words, instead of optimizing the performance of the continuous-time interpolation filter, the fractional-delay filter for each delay value is optimized, thereby achieving even better performance.